Facilitating role of biogenetic schwertmannite in the reduction of Cr(VI) by sulfide and its mechanism

Highlights: ► Schwertmannite was obtained through an oxidation of FeSO4 by Acidithiobacillus ferrooxidans LX5 cell suspensions. ► Schwertmannite can markedly accelerate the removal of Cr(VI) by sulfide. ► The catalysis is ascribed to the activated Fe(III) on schwertmannite serving as a "bridge" in the transportation of electrons between sulfide and Cr(VI). - Abstract: The efficient conversion of Cr(VI) to Cr(III) has attracted an increasing concern in recent years owing to its threat to the environment. In the present paper, the catalytic role of biogenetic schwertmannite in the reduction of Cr(VI) by sulfide and its mechanism were investigated under different conditions through batch experiments. The results demonstrated that schwertmannite markedly accelerated the removal of Cr(VI) by sulfide, and the rates of the reaction were enhanced by 11, 8 and 6 times, respectively at pH 7.5, 8.0 and 8.8 as compared with control (no schwertmannite). In addition, the conversion of Cr(VI) into Cr(III) increased with schwertmannite loading and temperature. However, the facilitating role of schwertmannite in the reduction of Cr(VI) by sulfide was markedly suppressed by an introduction of F−, a complex agent for Fe(III). It is concluded that the catalysis of schwertmannite results from the activated Fe(III) on the surface of schwertmannite, serving as a "bridge" in the transportation of electrons between sulfide and Cr(VI), and leading to the improving reduction of Cr(VI) by sulfide.